Since commercial purging compounds were introduced in the 1980’s, there have been great improvements made to increase the efficiency of these compounds. Slowly the resistance to using a commercial purging compound over in-house solutions is being overcome as the rising cost of resin prompts companies producing all categories of plastics to seek out cost-effective alternatives.
There is no question that purging is necessary between color changes and it is a vital part of a good preventative maintenance plan. Without adequate purging, money is lost due to contamination from color streaks when changing to another color, as well as stubborn black specks that result from carbon buildup when a residual resins or color adheres to the internal surfaces of a machine that does not have adequate turbulence to automatically remove color and residual resins. Over time residual carbon that builds within the machine in the manifold channels and screw surfaces breaks off and contaminates the finished product. Prior to the introduction of commercial purging compounds, these black specks could be minimized by running regrind or virgin resin through the machine. This is a costly procedure in both the waste of resin and the valuable downtime such a procedure requires.
Running regrind or virgin resin to purge a machine is not efficient. The large amount of waste and excessive loss of production time attempts to displace the contamination from residual colors, resins or carbon. The variability between materials and the operators running the procedure does not produce consistent results. Commercial purging formulas offer major advantages over in-house options that depend on re-grinds or virgin resins.
Advantages of Commercial Purging Formulas
Shorter Downtime – Unlike the use of re-grinds, commercial purging compounds are designed to efficiently clean with one pass through molding equipment when used in a regular maintenance plan recommended by most suppliers. This means far shorter downtime and increased efficiency from machinery and workers. Reduced Waste – With resin costs rising you can dramatically reduce waste by not using virgin resin to purge color or attempt to reduce carbon deposits. Efficient purging of carbon that leads to black specks means less contamination in finished products.
Consistency in Producing Quality Products – By following the recommendations provided by your commercial purging compound representative to properly use the purging compound that is right for your applications, you will consistently see an increase in savings and produce quality products.
Lower Maintenance and Purging Cost – When calculating the cost of machine maintenance, take into consideration the cost of replacing parts due to the inefficient cleaning of the machine when using in-house solutions, the downtime, the lost production and the waste due to contaminated products, then compare the cost-per- purge to see the significant savings that commercial purging products offer.
Four Types of Commercial Purging Compounds Available
With four major choices in commercial purging compounds available, it is important to understand the differences when deciding which compound is right for your machines and applications.
• Abrasive-Mechanical Compounds – Mechanical purging compounds such as those marketed by type A, use abrasive materials like talc, rock, glass, or calcium to mechanically scrub the insides of the barrels and screws to remove residual resin and color deposits. While efficient when used consistently, over time the abrasive action can cause excessive wear and pitting that may require premature parts replacement.
• Non-Abrasive Mechanical Compounds – type D, produces a non-abrasive purging compound. These compounds are based on a soft-molten product that uses viscosity to flush residual color as well as carbon deposits from molding equipment. Other types of non-abrasive compounds use a granular formula similar to the soft-scrub concept in some bathroom cleaners. The outer layer of the granules soften but a center remains solid to scrub away residual carbon buildup with less abrasive wear on screws and barrels. These products vary in their performance due to the adjustments needed in temperature and RPM speed needed to effectively purge the compound from the system.
• Chemical – Type U and other chemical purging agents rely on a chemical compound that changes the viscosity of residual resin so it is easier to purge from the molders. These compounds are usually supplied as concentrates that must be mixed into resin in controlled stages or manually measured and blended with carrier resin. The accuracy in which the concentrate is measured, mixed, and blended will affect the final results and efficiency of the purge. Too much does not improve overall results but increases the cost per purge, while not enough concentrate improperly blended will not yield the desired results. Most chemical purging compounds require soak time for full chemical reaction to occur. This increases downtime. The compound must then be heated to the correct temperature to allow the chemical reaction to readily flush from the system. Unless manufacturer’s directions are followed closely, fumes and ammonia could be released posing a hazard to equipment and workers. The shelf- life of chemical purging compounds is often limited. Since most chemical purging compounds are not heat stable these formulas cannot be left in the barrel during shutdown without resulting in carbon build up during start up. Because chemical compounds can contribute to carbon buildup and lake an abrasive action, they are not the best choice to remove carbon buildup.
• Chemical-Mechanical Combinations – Purge Right commercial purging compounds are a hybrid combination using both mechanical and chemical formulation that combines the best of both methods. Purge Right comes in a dry form and does not require mixing or soak time. The chemical component of the formulation in hybrid combinations change the viscosity of the resin while the addition of abrasive agents provides a mechanical scrubbing action to help decrease carbon buildup that leads to black specks. With consistent use in a preventative maintenance program, the unique formulation found in Purge right products can help prevent or decrease carbon build up in any molding equipment. Older machines or those where routine maintenance and purging has not been done in a timely fashion may require additional purging or follow up manual cleaning to completely remove carbon deposits from the screws.
Several important factors should be considered when choosing a commercial purging compound.
In addition to the cost of commercial purge compounds, you should also calculate the per-purge costs.
Per-purge cost includes such things as:
• Amount of compound needed per purge
• Amount and price of post-purge resin wasted
Purging time varies from company to company. Your purge time will depend on such things as the type of equipment you run; applications used; colors; resin type as well as resin viscosity before, during, and after purging; the size of the machinery as well as the age of the machinery; and the previous level of maintenance.
Your commercial purge compound representative can answer any questions and assist you in selecting the best purging compound for your machine and applications.